Impeller for axial flow pump

ABSTRACT

A rotor for an axial-flow blood pump has blades projecting outwardly from a hub and channels between the blades. The blades incorporate hydrodynamic bearing surfaces capable of suspending the rotor during operation. The rotor has a configuration which provides superior pumping action and reduced shear of blood passing through the pump. The forwardly facing pressure surfaces of the blades may include outflow corner surface at their downstream ends. The outflow corner surfaces desirably slope rearwardly and intersect the rearwardly-facing suction surfaces of the blades at outflow ends of the blades.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the filing dates of U.S.Provisional Patent Application Nos. 61/865,672, filed Aug. 14, 2013, and62/013,271, filed Jun. 17, 2014, the disclosures of which are herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to rotors for use in blood pumps and toblood pumps having such rotors.

Implantable blood pumps are employed as ventricular assist devices toaid the functioning of a diseased heart in a human patient or non-humananimal subject. When a blood pump is employed as a left ventricularassist device or “LVAD,” an inlet of the pump communicates with the leftventricle of the patient's heart, whereas the outlet of the pumpcommunicates with the aorta downstream of the aortic valve. Thus, thepump acts in parallel with the patient's left ventricle to impel bloodfrom the ventricle into the aorta. A pump used as an LVAD in a typicalhuman subject should be capable of providing substantial blood flow as,for example, a few liters per minute or more, against a pressure headcorresponding to the blood pressure of the subject. For example, in onetypical operating condition, an LVAD may pump 5 liters of blood perminute at 75 mmHg pressure head, i.e., a pressure at the outlet of thepump 75 mmHg higher than the pressure at the inlet.

Other blood pumps are applied as right ventricular assist devices. Inthis application, the inlet of the pump is connected to the rightventricle of the subject's heart, whereas the outlet of the pump isconnected to a pulmonary artery. Dual pumps can be used to provide bothleft and right ventricular assistance, or even as complete artificialhearts.

Implantable blood pumps should be compact so as to facilitate mountingthe pump within the patient's body. They should also provide highreliability in prolonged use within a patient, most typically years, oreven decades of service. An implantable blood pump also should beefficient so as to minimize the power required to operate the pump. Thisis particularly significant where, as in most applications, the pump ispowered by a portable battery or other portable power source carried onor in the patient's body. Moreover, the pump should be designed tominimize damage to the patient's blood. It should limit the amount ofblood subjected to relatively high sheer stresses as, for example, 150Pa or more, so as to minimize the damage to components of the blood.

One particularly desirable form of blood pump is disclosed in U.S. Pat.Nos. 7,699,508; 7,972,122; 8,007,254; and 8,419,609, all assigned to thepresent assignee. The disclosure of the foregoing patents isincorporated by reference herein. This type of blood pump is commonlyreferred to as a wide-blade axial flow blood pump. The pump includes ahousing having a bore and a rotor disposed within the bore. The rotorhas a hub extending along an axis and blades projecting outwardly awayfrom the hub. The blades are spaced apart from one another around theaxis so that the blades cooperatively define channels extending betweenadjacent blades. The channels are generally helical and extend along theaxis while also wrapping partially around the axis. The outer ends ofthe blades have tip surfaces facing in the outward direction, away fromthe axis. These tip surfaces have substantial area. The tip surfacesinclude hydrodynamic bearing surfaces. Typically, the rotor is magneticand includes two or more magnetic poles. Electrical coils are arrayedaround the housing. These coils are energized by an electrical powersource so as to provide a rotating magnetic field, which spins therotor. As the rotor spins, it impels blood axially in the housing, in adownstream direction along the axis. The hydrodynamic bearing surfacessupport the rotor on a film of blood disposed between the bearingsurfaces and the inner wall of the housing. Stated another way, thehydrodynamic bearings maintain the rotor coaxial with the bore andresist loads transverse to the axis of the rotor as, for example, loadsimposed by gravity or gyroscopic forces that can be created whenmovement of the patient tilts the pump. Magnetic interaction between therotor and the magnetic field applied by the coils resists axial movementof the rotor. In other variants, additional elements such as additionalmagnets or additional hydrodynamic bearings can be provided to resistaxial movement of the rotor relative to the housing.

Preferred wide-blade axial flow pumps according to the aforementionedpatents can be extraordinarily compact. For example, a pump suitable foruse as a left ventricular assist device may have a rotor on the order of0.379 inches (9.63 mm) in diameter and blades with an axial extent ofabout 0.5 inches (12.7 mm). The overall length of the rotor, includinghubs projecting upstream and downstream from the blades is about 0.86inches (21.8 mm). The housing has an inside diameter only slightlylarger than the diameter of the rotor. The electrical coils, housing,and rotor may be contained within an outer shell about 0.7 inches (18mm) in diameter and on the order of 1 inch (25 mm) long. In onearrangement, the outlet or downstream end of the housing is connected toa volute, which serves to connect the outlet end to an outflow cannula,whereas the inlet or upstream of the housing is inserted into thepatient's left ventricle through a small hole in the heart wall. Instill other arrangements, the entire pump may be positioned within theleft ventricle, and the outlet end of the housing may be connected to anoutflow cannula that projects through the aortic valve. See, U.S. PatentApplication Publication No. 20090203957 A1, the disclosure of which isincorporated herein.

The wide-blade axial flow blood pumps according to the aforementionedpatents and publication operate without wear. In operation, therotor—the only moving part of the pump—is suspended by the hydrodynamicbearings and magnetic fields and does not touch the housing. Such a pumphas theoretically infinite life. Moreover, preferred pumps according tothe aforementioned patents can operate for many years without thrombusformation.

Despite the significant progress in the art, still further improvementswould be desirable. In particular, it would be desirable to providegreater efficiency, improved pump performance, and reduced shear on theblood while still maintaining the advantages of the wide-blade axialflow blood pump. Such improvement poses a formidable engineeringchallenge. In a wide-blade axial flow pump of this type, the tipsurfaces of the rotor blades must provide sufficient area for effectivehydrodynamic bearings. The blades of the rotor must also have the volumeneeded to contain enough magnetic material to provide magnetic poleswith sufficient strength on the rotor. These constraints have limitedthe possible improvements in design of the rotor heretofore.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention provides an improved rotor for usein a blood pump. The rotor preferably has an axis extending in upstreamand downstream axial directions and a plurality of generally helicalblades extending from an inflow end of the rotor to an outflow end ofthe rotor. Desirably, the blades projecting outwardly away from the axisin a spanwise direction. The blades typically are coextensive in theaxial directions. The blades desirably are spaced apart from one anotherin a circumferential direction around the axis so as to define generallyhelical channels between adjacent ones of the blades. Each bladepreferably has a pressure surface facing in a forward circumferentialdirection, a suction surface facing in a rearward circumferentialdirection and a tip surface extending between the pressure and suctionsurfaces of the blade. Each channel desirably is bounded by the pressureside of one of the blades and by the suction side of a next adjacent oneof the blades. The tip surfaces of the blades most preferably definehydrodynamic bearing regions capable of suspending the rotor. Mostpreferably, the rotor is adapted to provide at least one of:

(a) at least 5 liters of blood flow at 75 mm Hg pressure head with aV₁₅₀ less than 25 mm³; and

(b) a specific blood flow rate of at least 50,000 mm/min at 75 mm Hgpressure head and a rotational speed of 15,000 revolutions per minute;and

(c) an average outflow angle less than 30 degrees.

Alternatively or additionally, the pressure surface of each said blademay include an outflow corner surface at the outflow end of the blade,the outflow corner surface extending over a major portion of thespanwise extent of the blade. Desirably, the outflow corner surfaceslopes in the rearward circumferential direction in the downstream axialdirection Most preferably, the outflow corner surface extends to within0.4 mm, and more pre of the suction surface of the blade at a downstreamextremity of the blade.

A further aspect of the present invention provides an improved bloodpump. The pump preferably includes a rotor as discussed above. The pumpdesirably has a housing defining a bore with an interior surface in theform of a surface of revolution, the rotor being disposed within thehousing with the axis of the rotor coaxial with the interior surface ofthe bore and with the interior surface of the bore closely overlying thetip surfaces of the blades. The pump desirably includes a drive arrangedto rotate the rotor about the axis. Yet another aspect of the presentinvention provides improved methods of pumping blood. A method accordingto this aspect of the invention desirably includes implanting a bloodpump as discussed above within the body of a patient, connecting thepump to the circulatory system of the patient and actuating the pump toassist blood flow within the circulatory system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a rotor in accordance with oneembodiment of the invention.

FIG. 2 is a perspective view of the rotor depicted in FIG. 1 from adifferent point of view.

FIG. 3 is an elevational view of the rotor depicted in FIGS. 1-2.

FIG. 4 is an end view of the rotor of FIGS. 1-3.

FIG. 5 is an opposite end view of the rotor depicted in FIGS. 1-4, andalso showing additional components of a pump in accordance with oneembodiment of the invention.

FIG. 6 is a partially schematic sectional view of the pump depicted inFIG. 5.

FIG. 7 is a further end view of the rotor shown in FIGS. 1-6.

FIGS. 8-13 are further elevational views of the rotor shown in FIGS. 1-7with portions of the rotor removed for clarity of illustration atdiameters indicated in FIG. 7.

FIG. 14 is a further elevational view of the rotor depicted in FIGS.1-13, depicting certain dimensions.

FIG. 15 is a sectional view taken along line A-A in FIG. 14, depictingadditional dimensions.

FIG. 16 is a partial sectional view of a pump incorporating the rotor ofFIGS. 1-15.

FIG. 17 is a fragmentary sectional view on an enlarged scale of the areaindicated at B in FIG. 16.

FIG. 18 is a graph depicting a property of the rotor of FIGS. 1-17 and acomparable property of the prior art rotor shown in FIGS. 19 and 20.

FIGS. 19 and 20 are perspective views depicting a rotor according to theprior art.

FIGS. 21, 22 and 23 are graphs showing certain operating characteristicsof a pump incorporating the rotor of FIGS. 1-17 and of a pumpincorporating the prior art rotor of FIGS. 19 and 20.

FIG. 24 is a diagrammatic developed view depicting a pair of rotorblades.

DETAILED DESCRIPTION

As used in this disclosure, the term “generally helical” refers to afeature which extends in the direction parallel to an axis and whichcurves in the circumferential direction around the axis over at least50% of its extent in the direction along the axis. The degree ofcurvature and pitch of a helical feature need not be uniform.

A rotor 30 according to one embodiment of the invention includes aunitary body incorporating a hub 32 extending along an axis 34.Directions along axis 34 are referred to herein as the “upstream” and“downstream” directions. Both such directions are also referred toherein as “axial” directions. The downstream direction is indicated ineach of FIGS. 1 and 2 by the arrow D; the upstream direction is theopposite direction.

A plurality of blades 36, in this instance 4 blades, project from thehub. Each blade 36 extends out of the hub in an outward radial or“spanwise” direction perpendicular to axis 30. Each blade also extendsin the lengthwise or axial directions over a portion of the axial extentof hub 32. The circumferential directions, i.e., rotational directionsaround axis 34, are indicated as the forward direction F and rearwarddirection R. Each blade defines a generally helical surface facing inthe forward direction F. Surface 38 is referred to herein as the“pressure” surface. Each blade also defines a surface 40 facing in theopposite or rearward direction R. Surface 40 is referred to herein asthe “suction” surface. Blades 36 are coextensive with one another in theaxial directions. Thus, as best seen in FIG. 3, the blades extend over acommon axial extent AX. In the particular example depicted, the axialextent of the blades is 0.500 inches (12.7 mm), and the maximum diameterD_(MAX) of the rotor, measured across the outermost extremities of theblades is approximately 0.379 inches (9.62 mm).

The blades are evenly spaced apart from one another around the axis, inthe forward and rearward circumferential directions. Thus, the bladesdefine a plurality of channels 42 extending between the upstream orinflow ends 37 and the downstream or outflow ends 39 of blades 36. Eachchannel 42 is bounded by the forwardly facing pressure surface 38 of oneblade and the rearwardly facing suction surface 40 of the next adjacentblade.

Each blade 36 has a tip surface 44 extending between the pressuresurface 38 and suction surface 40 of such blade. Each tip surface facesoutwardly away from axis 34 and defines the outermost extremity of theblade. Each tip surface includes a land surface 46. Land surface 46 isin the form of a part of a surface of revolution around central axis 34.In the particular embodiment depicted, the surface of revolution is acircular cylinder; so that the radius from the axis 34 to land surface46 is uniform over the entire extent of each land surface 46, suchradius being one-half of the maximum diameters D_(MAX) of the blades.Each tip surface 44 further includes an upstream hydrodynamic bearingsurface 48 and a downstream hydrodynamic bearing surface 50.

Each hydrodynamic bearing surface extends in the rearwardcircumferential direction from the pressure surface 38 of the blade. Asbest seen in FIGS. 1, 2, and 17, the upstream or inflow end bearingsurface 48 is recessed radially from the land area 46. The recess is ata maximum at the forward edge of the bearing surface, where the bearingsurface meets the pressure surface 38 of the blade. The recessdiminishes progressively in the rearward circumferential direction, sothat the bearing surface merges smoothly into the land area 46 at therearward edge of the bearing surface. The downstream bearing surface 50(FIGS. 1, 2, 3 and 14) of each blade has a similar configuration. In theparticular embodiment depicted, the forward edge of each bearing surfaceis recessed relative to the land area by a recess dimension RD (FIG. 17)of about 0.0030 to 0.0040 inches, i.e., 0.076 to 0.010 mm, mostpreferably 0.0035 inches (0.089 mm). As best seen in FIG. 14, the landarea 46 of each tip surface includes an inflow end region 54 borderingthe upstream or inflow end bearing surface 46 on the upstream sidethereof, a downstream or outflow end region 56 bordering the downstreambearing surface 50 on the downstream side thereof, a dividing wallregion 58 separating the upstream and downstream bearing surfaces fromone another, and a rearward edge region 60 extending along the junctureof the tip surface with the suction surface 40 of the blade. Thedimensions of certain features of the tip surface in the particularembodiment depicted are shown in inches in FIG. 14, along with angles ofcertain features relative to a plane perpendicular to the central axis34 of the rotor.

The tip surfaces 44 of the blades have a substantial circumferentialextent in the forward and rearward directions around central axis 34.Most preferably, the circumferential extent CET (FIG. 16) of each tipsurface 44 is greater than the circumferential extent CEC of eachchannel 42 measured at the outermost extremities of the blades. Thisrelationship between the circumferential extents of the tip surfaces andchannels preferably applies over a substantial portion of the axialextent of the blades and channels as, for example, at least about 30% ofsuch axial extent and more desirably over a major portion of such axialextent, i.e., at least about 50% of the axial extent of the blades andchannels. Stated another way, the aggregate area of the tip surfaces isgreater than the aggregate area of the channels, again as measured atthe outermost extremities of the blades. In the particular embodimentdepicted, the aggregate area of the tip surfaces (inclusive of thehydrodynamic bearing surfaces and land regions) is about 57% of the areaof a theoretical cylinder having a diameter equal to the maximumdiameter D_(MAX) (FIG. 3) of the blades and having a length equal to theaxial extent AX of the blades. Preferably, this ratio between the tipsurface area and the area of a theoretical solid surface of revolutioncorresponding to the tip surfaces is at least 0.50 and more preferablyat least about 0.55. The relatively large tip surfaces provide adequatearea for hydrodynamic bearing surfaces that are capable of suspendingthe rotor. As used in this disclosure, hydrodynamic bearing surfaces“capable of suspending the rotor” are hydrodynamic bearing surfacesthat, when the rotor is rotated about its axis in blood in a tubularhousing closely surrounding the tip surfaces at a rate required to pumpat least 5 liters per minute of blood at 75 mm pressure head, arecapable of maintaining the rotor coaxial with the housing so that therotor does not contact the housing due to radial movement, transverse tothe axis of the rotor, or due to tilting of the axis of the rotorrelative to the housing.

Hub 32 defines a floor surface 62 (FIGS. 1, 2, 3, and 5) within eachchannel 42. The floor surface faces radially outwardly, away from thecentral axis 34 of the rotor. As best seen in FIG. 6, hub 32 has aprogressively increasing diameter over at least a portion of its lengthwithin the axial extent of blades 36, and thus within the axial extentof channels 42. Thus, over a portion of the axial length of each channeladjacent the upstream (inflow) ends 37 of the blades, the floor surface62 defined by the outer surface of hub 32 slopes radially outwardly,away from central axis 34 in the downstream direction. A further portionof the hub within the axial extent of the blades and channels, butadjacent the downstream or outflow ends 39 of the blades and thedownstream ends of the channels, has a constant diameter. Thus, withinthis axial region of constant diameter, the floor surface of eachchannel does not slope relative to the axis 34.

The hub further defines an upstream end cone 64 projecting in theupstream or inflow direction beyond the upstream extremities 37 of theblades and tapering to a small radius. For example, the upstream endcone may project about (4.6 mm beyond the upstream extremities of theblades. Likewise, the hub includes a downstream end cone 66 projectingabout 0.180 inches 4.6 mm downstream from the downstream extremities 39of the blades.

The pressure surface 38 of each blade includes an outflow corner surface70 forming the downstream extremity of the pressure surface. The outflowcorner surface has a substantial helix angle, so that the outflow cornersurface 70 slopes in the rearward circumferential direction towards thedownstream extremity of the blade.

As used in this disclosure with reference to a helical surface, theterms “pitch angle” and “helix angle,” each mean the angle between aline tangent to the helical surface and the central axis 34. The pitchangle or helix angle is the compliment of the lead angle, i.e., theangle between a line tangent to the surface and a plane perpendicular tothe axis 34. Notably, the outflow corner surface 70 extends to andintersects the suction surface 40 of the blade. Ideally, the outflowcorner surface intersects the suction surface at a sharp edge 72 (FIG.2). In practice, edge 72 is broken or rounded slightly to make the edgeless delicate. However, even with such rounding, the outflow cornersurface desirably extends to within about 0.4 mm of the suction surfaceat the downstream extremity of the blade at least in a region of theoutflow corner surface near the outer end of the blade, i.e., near thetip surface. More preferably, the outflow corner surface of each bladeextends to within about 0.15 mm of the suction surface of the blade overat least a major portion of the spanwise or radial extent of the blade.The lead angle of the outflow corner surface 70 as measured at variouspoints along the spanwise extent of the blade at the downstreamextremity of the blade (at edge 72) varies along the spanwise extent ofthe blade. This is depicted in FIGS. 7-12. Each of FIGS. 8-12 is a sideview of the rotor 30 with an outermost portion of the blades removed forclarity of illustration. Thus, FIG. 8, labeled “0.0035-A,” shows therotor with that portion lying outside of the circle labeled “0.0035-A”in FIG. 7 removed for clarity of illustration. The legend “0.0035-A”indicates that the portion removed has a depth or radial extent of0.0035 inches (0.09 mm) from the outer-most extent of the actualphysical blade, i.e., that circle 0.0035-A lies at a radius 0.0035inches smaller than the maximum radial extent of the blades. Likewise,FIG. 12, labeled “0.0835-E,” shows the rotor with portions lying outsideof the circle labeled “0.0835-E” in FIG. 7 removed for purposes ofillustration. This circle has a radius 0.0835 inches (2.12 mm) less thanthe maximum radius of the actual blades. As indicated by FIGS. 8-12, thelead angle of the outflow corner surface 70 decreases in the radiallyoutward or spanwise outward direction, away from axis 34. Thus, asindicated in FIG. 12, the lead angle is about 23.14 degrees near theinner end of the outflow corner surface 70. Near the outer end ofsurface 70, the lead angle is about 3.13 degrees as indicated in FIG. 8.In general, the lead angle of the outflow corner surface should be lessthan 25 degrees over its entire spanwise extent, and its lead angleshould decrease in the radially outward or spanwise direction, so thatthe lead angle is less than 10 degrees, and preferably less than 5degrees, at the outer end of the outflow corner surface.

As best seen in FIG. 3, the outflow corner surface 70 intersects the tipsurface 44 of the blade along an outer curve 74 and also defines a curve76 at the radially inner edge of the outflow corner surface. Curve 76diverges in the forward circumferential direction F (FIG. 3) from curve74. Thus, a theoretical vector V₇₀ (FIG. 2), pointing out of outflowcorner surface 70 and normal to such surface, has positive, non-zerocomponents in the radially outward direction, away from axis 34 and inthe downstream direction D.

The pressure surface 38 of each blade also includes a main region 78(FIG. 3) extending upstream from the outflow corner surface 70. Withinthis main region, the pressure surface is generally helical. The mainregion extends to a radiused edge 81 at the upstream or inflow extremity37 of the blade. Edge 81 extends in the spanwise or radial direction. Afillet 80 is provided at the juncture of the pressure surface 38 and thechannel floor surface 62. This fillet has a relatively small radius.This fillet occupies only a small portion of the radial or spanwiseextent of the blades and channels.

The suction surface 40 of each blade includes an outflow region 84adjacent the outflow or downstream extremity 39 of the blade. Theoutflow region 84 has a low pitch angle, desirably less than about 10degrees and more typically about 0 degrees. Within outflow region 84,the suction surface lies in a plane parallel or nearly parallel to thecentral axis 34 of the rotor. As best appreciated with reference to FIG.3, the outflow region 84 of the suction surface is aligned, in the axialdirection, with the outflow corner region 70 of the pressure surface onthe next adjacent blade forming the opposite wall of a channel. In thisregion, adjacent the downstream or outflow end extremities 39 of theblades, the channel bounded by the blade has a width or circumferentialextent that increases rapidly in the downstream direction, so that thecross-sectional area of the channel also increases rapidly. The suctionsurface 40 also has a main region with a helix angle larger than thehelix angle of the outflow region 84.

The suction surface 40 of each blade further includes an inflow endregion 88 (FIGS. 1, 3) extending to the upstream extremity of the blade.As best appreciated with reference to FIG. 3, the inflow end region 88of each blade has a progressively increasing helix angle (progressivelydecreasing lead angle). The surface of the inflow end region becomesnearly parallel to a plane perpendicular to the axis 34 as it approachesthe upstream extremity 37 of the blade. At each axial location withinthe axial extent of the inflow end regions of the suction surface, thehelix angle of the inflow end region of the suction surface is greaterthan the helix angle of the pressure surface. Within this axial extent,the suction surface (inflow end region 88) diverges from the pressuresurface 38 of the next adjacent blade. Thus, the width orcircumferential extent of each channel increases in the upstreamdirection throughout the axial extent of the inflow end regions 88. Theinflow end regions 88 terminate at a location 89 (FIG. 14) on theupstream extremity of the blade, where the helix angle reaches 90degrees and thus the lead angle reaches 0 degrees. This location 89 liesclose to the radiused edge 81 of the pressure surface. In the particularembodiment illustrated, the distance between location 89 and the peak ofradiused edge 81, measured at the outer end of the blade near the tipsurface 44 is 0.037 inches, i.e., 0.94 mm. Thus, the blade presents onlya very small flat surface 92 between the upstream end of its suctionsurface (the upstream end of inflow end region 88) and radius 81, wherethe suction surface joins the pressure surface.

A fillet 96 is provided at the juncture between the suction surface 40of each blade and the adjacent channel floor surface 62. In the mainregion 86 and inflow end region 88 of the suction surface, fillet 96 hasa relatively small and substantially constant radius as. However, in theoutflow end region 84 of the suction surface, the radius of the fillet96 increases progressively in the downstream direction. Thus, as seen inFIG. 15, the radius R₉₆ of fillet 96 at the downstream or outflowextremity 39 of each blade is a substantial portion of the spanwise orradial extent of the blade and also a substantial portion of thecircumferential width of the channel. Preferably, the radius R₉₆ of thisfillet at the downstream end of the blade is about 25% or more of thespanwise or radial extent of the blade (the radial distance from thechannel floor surface 62 to the tip surface 44 of the blade) andlikewise is about 25% or more of the width or circumferential extent ofthe channel. In the particular example shown in FIG. 15, the radius R₉₆of the fillet occupies about one-third of the circumferential extent ofthe floor surface 62 of the channel 42. This progressively wideningfillet 96 gives the downstream end of the channel the shape of a scoopand thus is referred to herein as an “outflow scoop fillet.”

Rotor 30 desirably includes magnetic poles. Thus, the rotor may beformed from a solid mass of a biocompatible, ferromagnetic alloy as, forexample, a platinum-cobalt alloy. The rotor may be magnetized usingconventional techniques so as to impart two opposite magnetic poles tothe rotor. Alternatively, the rotor may be formed primarily from anon-magnetic material with one or more permanent magnets embeddedtherein.

The configuration discussed above provides the rotor with channelshaving relatively large area at the inflow end narrowing progressivelyto a smaller cross-sectional area adjacent the middle of the axiallength of the rotor and growing to a very large cross-sectional areaadjacent the outflow end of the rotor. The aggregate cross-sectionalarea of the channels in the particular example of the rotor discussedabove is indicated by curve 100 in FIG. 18. The cross-sectional area ofthe channels at various points along the axial length of the rotor isshown in FIG. 18. The aggregate cross-sectional area versus axiallocation is also shown in Table I below. In FIG. 18, and in Table 1, theaxial location 0 is at the radiused edge 81 at the upstream or inflowextremity 37 of the blade, and the other axial locations are measuredfrom axial location 0.

TABLE I Aggregate Cross-Sectional Axial Location Area (4 Channels) [mm][mm²] 0 36.5821 0.508 32.0307 1.016 29.7236 1.524 27.8071 2.032 26.21672.54 24.9176 3.048 23.8325 3.556 22.9136 4.064 22.1539 4.572 21.54915.08 21.0746 5.588 20.7055 6.096 20.4211 6.604 20.2516 7.112 20.57587.62 20.9674 8.128 21.3947 8.636 22.185 9.144 23.6229 9.652 25.760110.16 28.6981 10.668 32.6238 11.176 37.9521 11.684 46.0128

The rotor according to the above-discussed embodiment of the presentinvention referred to in curve 100 of FIG. 18 has an inflow area (theaggregate area of the channels) at axial location 0 of 36.5821 mm². Thearea of a solid circle having the same diameter as the maximum diameterof the rotor (9.6266 mm) is 72.78 mm². Thus, the specific inflow area(the ratio of the aggregate inflow area of the channels to the area ofthe theoretical solid circle having the same diameter as the maximumdiameter of the blades of the rotor) is approximately 0.503. Desirably,the channels provide a specific inflow area of at least 0.44, preferably0.48, more preferably at least 0.5. The outflow area (the aggregatecross-sectional area of the channels at axial location 11.684 in TableI) is 46.0128 mm². Thus, the specific outflow area (ratio of aggregateoutflow area of the channels to the area of the theoretical circlediscussed above) is 0.632. Desirably, the channels provide a specificoutflow area of at least 0.47, preferably at least 0.55 and morepreferably at least 0.6. The ratio of the aggregate outflow area to theaggregate area of the channels at the location where the aggregate areais at a minimum (axial location 6.604 mm), hereinafter referred to asthe “outflow/min ratio,” is 2.253.

A comparable rotor according to the prior art is depicted in FIGS. 19and 20. FIGS. 19 and 20 are similar to FIGS. 1 and 2, respectively. Notethat the rotor according to the prior art does not have the outflowcorner surfaces extending to the suction surfaces as discussed above,and thus has substantial flat areas 201 disposed essentiallyperpendicular to the axis. The prior art rotor according to FIGS. 19 and20 was previously regarded in the art to a providing the best possiblecombination of pumping performance with reasonable shear and withadequate hydrodynamic bearing surface area to maintain the rotor inposition. The aggregate area of the channels in the prior art rotor, atthe same axial locations as in Table I above, is depicted in curve 102in FIG. 18 and shown in Table II below:

TABLE II Aggregate Cross-Sectional Axial Location Area (4 Channels) [mm][mm²] 0 29.9032 0.508 27.6112 1.016 25.6552 1.524 23.9183 2.032 22.50272.54 21.3136 3.048 20.2826 3.556 19.5393 4.064 18.6418 4.572 17.72545.08 17.1802 5.588 16.3937 6.096 15.8288 6.604 15.4733 7.112 15.84517.62 16.4472 8.128 17.1506 8.636 18.153 9.144 19.2145 9.652 20.533710.16 21.8014 10.668 23.6393 11.176 26.8451 11.684 32.4964

The specific inflow area for the prior art rotor of FIGS. 19 and 20 isapproximately 0.411, and the comparable specific outflow area for theprior art rotor is 0.446. The outflow/min ratio (the ratio of theaggregate outflow area to the aggregate area of the channels at thelocation where the aggregate area is at a minimum (axial location 6.604mm)) is 2.100. The rotor according to the embodiment of the presentinvention discussed above provides substantially increased inflow andoutflow areas, and a greater outflow/min ratio. Notably, the increasedinflow and outflow areas, and generally increased channelcross-sectional areas, are provided while still maintaining adequateareas on the tip surfaces to provide hydrodynamic bearings that willsupport the rotor in operation. Moreover, the advantageous channelconfigurations and areas in the embodiment according to the presentinvention discussed above are also provided while maintaining anadequate mass of material to provide proper magnetic interaction asdiscussed below.

A pump according to one embodiment of the present invention includes arotor 30 as discussed hereinabove with reference to FIGS. 1-17 inconjunction with a housing 110 defining an interior bore 112 (FIGS. 5,6). The interior bore closely surrounds the tip surfaces of the rotor.For example, the diameter of the interior bore may be about 0.089 mm toabout 0.121 mm larger than the maximum diameter D_(MAX) of the rotor, sothat the housing provides approximately 0.05 mm radial clearance fromthe land regions of the tip surfaces. A set of coils schematicallyindicated at 114 is arrayed around the exterior of the housing. Coils114 may be of conventional construction. Merely by way of example, thecoils may be provided as three sets of diametrically opposed coilsdisposed at equal spacings around the circumference of the housing. Thecoils are associated with a conventional ferromagnetic component,commonly referred to as a stator iron (not shown). A shell 116 surroundsthe coils, stator iron and housing. Because the rotor itself is verysmall, the shell also may be of small diameter as, for example, 21 mm orless, and preferably 18 mm or less.

In operation, with the pump implanted in the body of a human or otheranimal subject, and with the housing connected into the circulatorysystem as, for example, in the conventional manner for a ventricularassist device, coils 114 are actuated to provide a magnetic fielddirected transverse to the central axis 34 of the rotor and to causesuch field to rotate rapidly around the axis. The magnetized rotorrotates along with the rotating magnetic field. The rotation directionof the magnetic field is selected so that the rotor spins in the forwardcircumferential direction F (FIG. 1). The spinning rotor pumps the bloodin the downstream direction D shown in FIG. 6. The spinning rotor alsoimparts some angular momentum to the blood around the central axis 34 ofthe rotor. Optionally, the housing may include additional componentsschematically indicated at 67 for converting this angular momentum intoadditional pressure, as discussed in the patents and publicationsmentioned above. The pumping performance discussed below is determinedin a pump having such components. Such components may include stationaryvanes mounted within the housing downstream of the rotor, and may alsoinclude a volute having a generally spiral shape oriented in a planetransverse to the axis 34, such volute being connected to the downstreamend of the tubular housing shown. For example, if stationary vanes areused, they may be generally helical and may have a pitch directionopposite to the pitch direction of the blades.

Typically, the rotor spins at rotational speeds on the order of severalthousand revolutions per minute (“RPM”) as, for example, more than tenthousand RPM. Under these conditions, blood confined between thehydrodynamic bearing surfaces 48 and 50 of the rotor (FIGS. 1, 2) andthe wall defining bore 112 of the housing maintains the rotorsubstantially coaxial with the bore of the housing and maintains therotor out of contact with the bore wall.

The hydrodynamic bearings do not control the axial location of therotor. Rather, the rotor is maintained in position along the axis bymagnetic interaction with stator iron associated with coils 114. Thus,the rotor is levitated within the housing and is not in contact with anysolid surface during normal operation. The housing may be provided withsafety stops (not shown) to constrain the rotor against axial movement.However, these safety stops do not contact the rotor during normaloperation.

FIG. 21 depicts one comparison between the pumping performance of a pumphaving a rotor according to the embodiment of the present inventiondiscussed hereinabove with an identical pump having the prior art rotorshown in FIGS. 19 and 20. Curve 150 depicts the volume versus headrelationship for a pump incorporating the rotor of the present inventionoperating at 15,000 RPM, as determined by computational fluid dynamics.Curve 152 depicts the same relationship for an otherwise identical pumphaving the prior art rotor of FIGS. 19 and 20 operating at 18,000 RPM,also as determined by computational fluid dynamics. The pumpincorporating the rotor according to the present invention providesbetter performance, even though it is operating at a substantially lowerspeed. The superior performance of the pump and rotor according to thepresent invention are further shown by FIG. 23. The solid-line curveslabelled “original” in FIG. 23 represent performance of the pump havingthe prior art rotor of FIGS. 19 and 20 at the speed indicated for eachcurve. The dotted-line curves labelled “modified” represent performanceof the identical pump having the rotor according to the embodiment ofthe present invention discussed above. Note that for any given pressurehead, the pump and rotor according to the present invention provide moreflow when operated at the same speed or, alternatively, the same flowwhen operated at a lower speed. The curves of FIG. 23 represent actualflow measurements taken using a water/glycerol solution at 37 degrees C.and having a viscosity of 2.7 cP (centipoise) to simulate blood.

As used in this disclosure, the term “specific blood flow rate” refersto the ratio of (i) the flow rate of blood or of a fluid having aviscosity of 2.7 cP (centipoise) to (ii) the area of a circle having adiameter equal to the maximum diameter of the rotor. Desirably, a pumpand a rotor according to the present invention may have a specific bloodflow rate of at least 50,000 mm/min, more preferably at least 55,000mm/min or at least 60,000 mm/min, and most preferably at least 68,000mm/min at 75 mm Hg pressure head and a rotational speed of 15,000revolutions per minute.

Notably, for a given flow rate and a given pressure head, the pump androtor according to the present invention operate with substantially lessexposure of the blood to high shear conditions. This is shown in FIG.22. The vertical axis indicates the volume of blood in and around therotor (including blood between the rotor and the housing) which isexposed to a shear stress of 150 Pa or greater. This parameter isreferred to herein as “V₁₅₀.” Curve 160 represents V₁₅₀ for a pumphaving the prior art rotor of FIGS. 19 and 20 when operated at 18,000RPM. Curve 162 represents V150 for the pump having the rotor accordingto the above-described embodiment of the present invention, operated at15,000 RPM so as to deliver the same or greater pressure differential atthe same flow rates as the prior art pump.

Moreover, the pump according to the present invention uses less power toprovide a given flow rate. For example, the pump according to theabove-described embodiment of the present invention can pump bloodagainst a pressure differential of 75 mm Hg. using 0.96 Watts ofelectrical power for each liter per minute of flow rate. The identicalpump using the prior art rotor consumes 1.18 W/L/min under similarconditions.

The embodiments of the present invention can be varied in many ways. Forexample, the rotor can be made with different diameter, differentlength, different number of blades and channels, and the like. Also, therotor and housing need not be cylindrical. For example, the bore of thehousing may be conical, and the tip surfaces of the blades may also beconical. Also, individual physical features of the rotor and pumpdiscussed above may be omitted or varied.

Although the present invention is not limited by any theory ofoperation, the improved performance achieved by certain rotors accordingto the present invention can be understood with reference to a theorycommonly referred to as “velocity triangles.” FIG. 24 schematicallydepicts a pair of rotor blades 336 in a developed view, as they wouldappear if the rotor was planar rather than cylindrical. The rotationalspeed of the rotor is indicated by arrow ω, so that a point 301 on anupstream or inlet end of the rotor disposed at a first radius from theaxis is moving with the velocity shown by vector U₁, and a point 303 onthe downstream or outlet end of the rotor at the second radius from theaxis has a velocity vector U₂. Both U₁ and U₁ are directed perpendicularto the axis of rotation of the rotor. The fluid flowing into the rotorat a rate Q has a velocity vector C₁ relative to the housing of thepump, referred to herein as the “absolute” inflow velocity. The velocityof the incoming fluid relative to point 301 on the rotor blade is shownby vector W1 and referred to herein as the “relative” inflow velocity.The angle β₁ between the relative inflow velocity W1 and a plane 305perpendicular to the axis of rotation is referred to herein as the“inflow angle.” Similarly, the fluid flowing out of the rotor has avelocity vector C₂ relative to the housing of the pump, referred toherein as the “absolute” outflow velocity. The velocity of the outgoingfluid relative to point 303 on the rotor blade is shown by vector W₂ andreferred to herein as the “relative” outflow velocity. The angle β₂between the relative outflow velocity W₂ and a plane 307 perpendicularto the axis of rotation is referred to herein as the “outflow angle.” Intheory, the head H developed by the pump is given by:H=(u ₂ ² −u ₁ ² +w ₁ ² −w ₂ ² +c ₂ ² −c ₁ ²)/2gwhere u1, u2, w1, w2, c1, and c2 are the magnitudes of the correspondingvectors as discussed above and g is the gravitational acceleration.

Various factors in the design of the rotor can influence the vectors andthus the theoretical head. Although, here again, the present inventionis not limited by any theory of operation, it is believed that theimproved performance achieved by preferred rotors according to thepresent invention is related to a decrease in the outflow angle β₂achieved by such rotors. Thus, the preferred rotors according to thepresent invention desirably provide an average outflow angle β₂ lessthan 30 degrees, and preferably about 25 degrees. By comparison, therotors of the prior art shown in FIGS. 19 and 20 provide an averageoutflow angle of about 45 degrees. The preferred rotors according to thepresent invention desirably have an average inflow angle β₁ less than 30degrees, and preferably about 25 degrees or less, in contrast to theaverage inflow angle of about 45 degrees in the same prior art rotors.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be used.

For example, the rotor of FIGS. 1-17 includes numerous features, each ofwhich contributes to the improved performance achieved by the rotor andby the pump incorporating the rotor. One or more of these features maybe omitted. Merely by way of example, the outflow corner surface 70(FIG. 1) may be used without the outflow scoop fillet 96 (FIGS. 4, 15)and vice-versa. Either or both of these features may be used without theinflow end region 88 (FIG. 1) of the suction surface, and vice-versa.

The invention claimed is:
 1. A rotor for an axial flow blood pump, therotor having an axis extending in upstream and downstream axialdirections and a plurality of generally helical blades extending from aninflow end of the rotor to an outflow end of the rotor, the bladesprojecting outwardly away from the axis in outward spanwise directions,the blades being coextensive in the axial directions and spaced apartfrom one another in a circumferential direction around the axis so as todefine generally helical channels between adjacent ones of the blades,each said blade having a pressure surface facing in a forwardcircumferential direction, a suction surface facing in a rearwardcircumferential direction and a tip surface extending between thepressure and suction surfaces of the blade, each said channel beingbounded by the pressure surface of one of said blades and the suctionsurface of a next adjacent one of said blades, the tip surfaces of theblades defining hydrodynamic bearing surfaces capable of suspending therotor, wherein the pressure surface of each said blade includes anoutflow corner surface at the outflow end of that blade, the outflowcorner surface of each said blade extending over a major portion of theextent of that blade in the outward spanwise direction of that blade,the outflow corner surface sloping rearwardly in the downstream axialdirection and extending to within 0.4 mm of the suction surface of theblade at a downstream extremity of the blade adjacent the tip surface ofthe blade, and wherein, over at least a major portion of the outflowcorner surface, a vector normal to the outflow corner surface haspositive, non-zero components in the outward spanwise direction of thatblade direction and in the downstream axial direction.
 2. A rotor asclaimed in claim 1 wherein the outflow corner surface of each said bladeextends to within 0.15 mm of the suction surface of that blade over atleast a major portion of the extent in the outward spanwise direction ofthat blade.
 3. A rotor as claimed in claim 1 wherein each said outflowcorner surface defines an outer edge at the tip surface of a blade andan inner edge extending along an inner curve at an inner end of theoutflow corner surface, the inner curve sloping rearwardly in thedownstream axial direction, the inner curve diverging in the forwardcircumferential direction from the outer edge.
 4. A rotor as claimed inclaim 1 wherein the rotor includes a central hub, the blades projectingoutwardly from the hub and the hub defining outwardly-facing floorsurfaces, each such floor surface bounding one of said channels, therotor further comprising fillets adjacent the outflow end of the rotor,each said fillet joining the suction surface of one said blade and thefloor surface of the channel bounded by such suction surface, the fillethaving a radius which increases progressively in the downstreamdirection.
 5. A rotor as claimed in claim 4 wherein the radius of eachsaid fillet at the outflow end of the rotor is at least about 25% of theextent in the outward spanwise direction of the suction surface joinedby that fillet.
 6. A rotor as claimed in claim 4 or claim 5 wherein thesuction surface of each said blade has a pitch angle of less than 10degrees in a region adjacent the outflow end of the rotor where thefillet is provided.
 7. A rotor as claimed in claim 4 wherein the hub hasa diameter which increases progressively in the downstream directionover at least a portion of the axial extent of the channels, whereby thefloor surfaces bounding the channels slope outwardly in the downstreamdirection over at least such portion of the axial extent.
 8. A rotor asclaimed in claim 1 wherein, in each said blade, the suction surfaceincludes an inflow end region at the inflow end of the blade, the inflowend region having a pitch angle of less than 90 degrees over its entireextent, the inflow end region extending to within 1 mm of the pressuresurface of the blade at and adjacent the tip surface of the blade.
 9. Arotor as claimed in claim 8 wherein the inflow end region of each saidblade extends to within 1 mm of the pressure surface over at least amajor portion of the extent of that blade in the outward spanwisedirection.
 10. A rotor as claimed in claim 9 wherein the suction surfaceof each blade includes a middle region remote from the inflow andoutflow ends of the blade, the suction surface having a first pitchangle in the middle region, the inflow end region of the suction surfacehaving a pitch angle greater than the first pitch angle.
 11. A rotor asclaimed in claim 9 wherein the pitch angle of the inflow end region ofeach said blade increases progressively toward an upstream extremity ofthe blade.
 12. A rotor as claimed in claim 8 wherein, at each axiallocation within the axial extent of the inflow end regions, pitch anglesof the inflow end regions are greater than pitch angles of the pressuresurfaces of the blades, whereby the inflow end region of the suctionsurface bounding each said channel diverges from the pressure surfacebounding that channel and each said channel widens circumferentially inthe upstream direction.
 13. A rotor as claimed in claim 1 wherein theblades have inflow edges at the inflow end, and wherein the inflow edgesof the blades slope in the downstream axial direction over a majorportion of the span of the blades.
 14. A rotor as claimed in claim 1wherein the tip surfaces of the blades have an aggregate area of atleast 50% of the area of a solid surface of revolution about the axishaving radius at each location along the axis corresponding to a maximumradius of the blades at the same location along the axis.
 15. A rotor asclaimed in claim 14 wherein the tip surface of each said blade includesa land surface in the form of a part of a surface of revolution aboutthe axis and one or more of said hydrodynamic bearing surfaces, eachsaid hydrodynamic bearing surface having a leading edge at the pressuresurface of the blade, each said leading edge being recessed inwardlyfrom the land surface, and each said hydrodynamic bearing surfacesloping outwardly in a rearward circumferential direction away from theleading edge.
 16. A rotor as claimed in claim 15 wherein the leadingedge of each said hydrodynamic bearing surface is recessed radiallyinwardly from the surface of revolution by 0.076 to 0.010 mm.
 17. Arotor as claimed in claim 15 wherein the hydrodynamic bearing surfacesof each blade include a front hydrodynamic bearing surface adjacent theinflow end of the rotor and a rear hydrodynamic bearing surface adjacentthe outflow end of the rotor, the front and rear hydrodynamic bearingsurfaces being separated from one another by an intermediate walldefining a portion of the surface of revolution.
 18. A rotor as claimedin claim 17 wherein a ratio of the area of the front hydrodynamicbearing surface to the area of the rear hydrodynamic bearing surface isless than 1.13.
 19. A rotor as claimed in claim 18 wherein said ratio is1.10.
 20. A rotor as claimed claim 1 wherein each said blade has aconstant extent in the outward spanwise direction over a majority of itsaxial extent.
 21. A rotor as claimed in claim 1 wherein the suctionsurface (40) of each blade includes an outflow region (84) adjacent adownstream extremity (39) of the blade, and the outflow region (84) hasa pitch angle less than 10 degrees.
 22. A blood pump comprising a rotoras claimed in claim 1 and a housing defining a bore with an interiorsurface in the form of a surface of revolution, the rotor being disposedwithin the housing with the axis of the rotor coaxial with the interiorsurface of the bore and with the interior surface of the bore closelyoverlying the tip surfaces of the blades, the pump further comprising adrive arranged to rotate the rotor about the axis.
 23. A blood pump asclaimed in claim 22 wherein, in operation, the rotor is suspended withinthe bore and maintained out of contact with the interior surface of thehousing by operation of the hydrodynamic bearing surfaces.
 24. A bloodpump as claimed in claim 23 wherein the rotor includes a plurality ofmagnetic poles and the drive includes a plurality of coils spaced apartfrom one another around a circumference of the bore and an electricalcircuit arranged to energize the coils so as to produce a rotatingmagnetic field within the bore.
 25. A blood pump as claimed in claim 24having a power-to-flow ratio of less than 1.05 watts per liter perminute at a pressure head of 75 mm Hg.
 26. A method of pumping bloodcomprising implanting a blood pump as claimed in claim 22 within a bodyof a patient, connecting the pump to the circulatory system of thepatient, and actuating the pump to assist blood flow within thecirculatory system.
 27. A method as claimed in claim 26 wherein the stepof connecting the blood pump includes placing an inlet of the pump incommunication with a ventricle of the patient's heart and placing anoutlet of the pump in communication with an artery of the patient.